As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices. Among them is a lithium secondary battery having high energy density and high discharge voltage, on which much research has been carried out and which is now commercially and widely used.
Depending upon kinds of external devices in which the secondary batteries are used, the secondary batteries may be used in the form of a single battery or in the form of a battery pack having a plurality of unit cells electrically connected with each other. For example, small-sized devices, such as mobile phones, can be operated for a predetermined period of time with the output and the capacity of one battery. On the other hand, a battery pack needs to be used in medium- or large-sized devices, such as laptop computers, electric vehicles, and hybrid electric vehicles, because high output and large capacity are necessary for the medium- or large-sized devices.
The battery pack is a battery structure in which a plurality of unit cells are electrically connected in series and/or in parallel with each other. It is very important for the battery pack to maintain the stable arrangement structure of the unit cells such that the unit cells can be sequentially connected with each other.
Especially, cylindrical secondary batteries, which are used as the unit cells of the battery pack, have great difficulty in maintaining the arrangement structure due to the appearance characteristics thereof. For this reason, the use of additional fixing members is necessary for the cylindrical secondary batteries. For example, there has been normally used a method of arranging cylindrical secondary batteries and fixing the cylindrical secondary batteries with fixing tape or coating cylindrical secondary batteries with plastic so as to maintain the arrangement structure of the cylindrical secondary batteries. According to circumstances, double-sided adhesive tape may be attached between the unit cells so as to further increase the coupling force between the unit cells.
However, the strength of the fixing tape and the plastic coating is small, and therefore, the possibility is very high that the fixing tape and the plastic coating are damaged or broken. Consequently, the fixing tape and the plastic coating are not suitable for stably maintaining the arrangement structure of the unit cells. In addition, when the battery pack is constructed using the fixing tape or the double-sided adhesive tape, a process for detaching and attaching the fixing tape or the double-sided adhesive tape is complicated and troublesome, which is a factor to increase the time necessary for constructing the battery pack.
Furthermore, it is required that the battery pack have various outputs and capacities by adjusting the number of the unit cells connected with each other depending upon the specification of a device in which the battery pack is used. In the cylindrical unit cells, however, a fixing member for maintaining the arrangement structure of the cylindrical unit cells is requisite. Consequently, there is a problem that various kinds of fixing members must be manufactured depending upon the number of unit cells to be assembled when the battery pack is constructed with the cylindrical unit cells. In order to solve this problem, a unit fixing member may be manufactured such that one or more unit cells can mounted to the unit fixing member, and a plurality of unit fixing members may be connected with each other to construct a battery pack. In this case, however, it is necessary that the unit fixing members be coupled with each other depending upon desired output and capacity of the battery pack.
Consequently, the necessity of a technology for fundamentally solving the above-mentioned problem is very high.